(1) Field of the Invention
The present invention relates generally to radio frequency (RF) linear power amplifier bias networks and, more particularly, to a bias network for minimizing distortion products normally associated with bipolar transistor based RF power amplifiers.
(2) Description of the Prior Art
An important goal associated with design of bipolar transistor based linear amplifiers includes minimizing the introduction of distortion products. It is known that load impedance can be optimized for minimum distortion. Optimization of just the load impedance, however, is often undesirable since the output power and efficiency generally are reduced. It is also known that any bias network must supply the correct amount of bipolar transistor base current to prevent or minimize distortion. Two trends associated with bipolar transistor base current must be reconciled to produce a linear amplifier with minimum amplitude modulation (AM) distortion, e.g. AM-to-AM. For example, the bias current required by a bipolar transistor in class B operation increases as the square root of the power. Further, the base current, and thus the collector current increases exponentially with increasing base-emitter voltage. Any reduction in distortion products will allow a linear amplifier to be operated closer to saturation, thereby improving the efficiency.
When a linear amplifier bias point is chosen very close to a class B mode, efficiency can be improved. This condition, however, places a heavy demand on the associated bias network to supply a large range of bias currents as the linear amplifier power requirements vary. Two approaches have been used in the art to provide the requisite bias network. First, a resistive bias network has been used where the base current is supplied through a bias resistor. Second, an active bias network has been used where an emitter follower transistor is used to provide a low impedance bias supply. The resistive bias approach provides limited bias current control over power. For example, if the resistor is small, temperature variations will cause unacceptable fluctuations in the quiescent current unless the bias network supply voltage also changes with temperature. If the resistor is large, the linear amplifier will be have insufficient bias current at high drive levels or have a large quiescent bias current which is undesirable. The active bias network allows an RF device to draw varying amounts of bias current depending upon the RF drive while maintaining a low quiescent level. The foregoing bias networks, therefore, can affect the linearity of an RF amplifier.
As stated above, one measure of linearity is AM-to-AM distortion due to RF amplifier gain changes that occur as the RF amplifier power level changes. The gain of an amplifier with resistive biasing will decrease as the power increases since the bias resistor will not pass the increased base current. Amplifiers with active biasing, however, will exhibit gain expansion since the effective bias current will increase at a larger rate than that required as the power is increased. This condition occurs because the average impedance looking back into the emitter of the bias current supply transistor decreases as the current increases.
In view of the above, a temperature compensated amplifier quiescent current is desirable since it helps maintain linearity and efficiency over the desired operating range of the amplifier. One technique that has been used to produce temperature compensation at a specific bias voltage includes a combination of resistive biasing and active biasing referred to in the art as "buffered passive bias." The buffered passive bias scheme reduces the current that must be supplied by the bias network voltage source. Another technique that has often been used to produce temperature compensation includes a current mirror bias network. The current mirror bias network provides bias current control over a wide temperature range, but requires higher levels of current from the bias network voltage source. In one case, thermal variations in the amplifier output transistor quiescent current, when using a current mirror bias network, track current changes through a collector bias resistor as the base-emitter voltage associated with the current mirror transistor and amplifier transistor change over temperature. If the bias network voltage is large compared to the base-emitter voltages, then the quiescent current will not change much over temperature.
The above techniques, familiar to those skilled in the art of linear amplifiers, affect the AM-to-AM linearity performance of the amplifier. As known in the art, amplifier performance limitations are affected by impedance variations seen looking back into the bias and RF matching networks. In one known embodiment, the amplifier output transistor collector current varies exponentially with its base-emitter voltage, as stated above. Therefore, a large RF impedance at the amplifier output transistor base is desirable for linearity since it will behave more like a constant current source. Use of a large RF impedance, however, is not desirable to achieve optimum energy transfer. One known technique that addresses the foregoing problems includes setting the value of an input RF coupling capacitor to the requisite value to achieve desired RF performance with the understanding that a higher impedance (smaller capacitor value) will achieve better linearity.
In class B operation, one requirement placed upon the associated bias network includes metering charge into an input RF coupling capacitor on the negative portion of the RF cycle at a rate that increases as the square root of the RF power. This charge is then pumped into the amplifier transistor base during the positive portion of the RF cycle. As stated above, a factor in controlling amplifier linearity is the impedance of the bias network. Other than the resistive bias technique, known biasing techniques discussed above generally have impedances that are too low. This characteristic generally tends to supply charge (current) to the input RF coupling capacitor discussed above at a higher rate than needed as the power increases and thus produces unwanted gain expansion. While the linearity performance of a resistive bias amplifier can be optimal, such techniques generally require excessive bias current from the bias network voltage source.
Thus, there remains a need for a new and improved bias network suitable for use with bipolar transistor power amplifiers and that effectively minimizes distortion products to achieve optimum linearity while substantially preserving efficiency.